1. Field of the Invention
The present invention relates to a control circuit of a light emitting element such as a semiconductor laser diode, particularly to a constitution of monitoring light emitting states of a plurality of light emitting elements by a single light receiving element and controlling individual light emitting elements by monitor current thereof.
2. Description of the Related Art
Conventionally, a discrete element integrated with a light emitting element and a light receiving element, for example, a discrete element integrated with a laser diode and a photodiode, is constructed by a pair constitution of one element of laser diode coupled with one element of photodiode (hereinafter, such an element is referred to as 1LD-1PD element). Therefore, it is general that an auto power control circuit (hereinafter, referred to as APC circuit) of a laser diode carries out light control with respect to 1LD-1PD element. For example, as shown by FIG. 6, a laser diode LD is made to emit light by generating drive current ILD from a driver circuit X1. When the laser diode LD emits light, monitor current IPD is generated at a photodiode PD and the current is converted into voltage by a current/voltage converting portion X2. There is calculated a difference between the voltage and reference voltage by an error amplifier X3 and a difference voltage component thereof is subjected to voltage to current conversion by the driver circuit X1 to thereby generate the drive current ILD. By such an APC loop, compensation of error current of the drive current ILD is carried out.
According to the constitution of FIG. 6, when a discrete element paired with two or more of laser diodes and one photodiode (hereinafter, referred to as 2LD-1PD element) is controlled, although drive current of two laser diodes cannot be controlled by single monitor current, there has been carried out a trial of an APC circuit for controlling drive current of two laser diodes by single monitor current, for example, there is provided a light amount control apparatus in Japanese Patent Laid-Open No. 209545/1998.
According thereto, as shown by FIG. 7, monitor current of a photodiode PD1 constituting a 2LD-1PD element along with laser diodes LD1 and LD2, is divided uniformly by a current dividing circuit DIV, distributed to amplifiers Y1 and Y2, respectively subjected to current-voltage conversion by the amplifiers Y1 and Y2 and output voltage of these is compared with reference voltage by comparators Y3 and Y4 in the comparators Y3 and Y4. When output voltage of the amplifiers Y1 and Y2 is lower than the reference voltage, the logical level of the output is determined as xe2x80x9cLxe2x80x9d and when the output voltage of the amplifiers Y1 and Y2 is higher than the reference voltage, the logical level is determined as xe2x80x9cHxe2x80x9d. Outputs of the comparators Y3 and Y4 are respectively provided to sample hold control circuits (hereinafter, referred to as SH control circuits) Y5 and Y6. The SH control circuits Y5 and Y6 are provided with up/down counters, not illustrated, when an enable state is brought about, the outputs are upcounted or downcounted in accordance with the logical levels of the outputs of the comparators Y3 and Y4 and when a disable state is brought about, count values are held. The SH control circuits Y5 and Y6 upcount the outputs when the outputs of the comparators Y3 and Y4 are at xe2x80x9cLxe2x80x9d and downcount the outputs when the outputs are xe2x80x9cHxe2x80x9d. The driver circuits Y7 and Y8 generate drive current ILD1 and ILD2 in accordance with count values of the SH control circuits Y5 and Y6. Single monitor current is divided in two by the current dividing circuit DIV and there is carried out compensation of error current of the drive current ILD1 and ILD2 similar to the constitution shown in FIG. 6 respectively by an APC loop including the amplifier Y1, the comparator Y3, the SH control circuit Y5 and the driver circuit Y7 and an APC loop including the amplifier Y2, the comparator Y4, the SH control circuit Y6 and the driver circuit Y8.
However, according to the constitution shown in FIG. 7, in the current dividing circuit DIV for dividing the monitor current of the photodiode, emitters of PNP bipolar transistors TR1, TR2 and TR3 are respectively connected to a common power source terminal via resistors R1, R2 and R3, a base and a collector of the transistor TR1 are connected to the photodiode and bases of the transistors TR2 and TR3 are connected to the base of the transistor TR1 to thereby constitute a current mirror circuit. Bias to the bases of the transistors TR2 and TR3 is constituted only by the monitor current of the photodiode and therefore, when the monitor current of the photodiode is reduced, drive function at post stages of the amplifiers Y1 and Y2 is deteriorated and an operational frequency band is attenuated. Therefore, it is difficult to make low consumption formation of the APC circuits and high frequency formation of operational frequency band compatible with each other.
Further, the amplifiers Y1 and Y2 are respectively provided with operational amplifiers Y9 and Y10, positive inputs thereof are connected to the ground via resistors R4 and R5 and connected to the current dividing circuit DIV via diodes D1 and D2 and variable resistors RV1 and RV2 and negative inputs thereof are connected to the voltage dividing circuit DIV via the diodes D1 and D2 and resistors R6 and R7. Input voltage to the operational amplifiers Y9 and Y10 is derived from voltage drop by the resistors connected to the ground and current-voltage conversion is carried out with a ground level as a reference. Therefore, it is difficult to fit the positive and negative inputs to input ranges capable of realizing optimum operation of the operational amplifiers Y9 and Y10 to thereby hamper high degree APCs. That is, when the inputs are received from the ground, generally, an input stage of the operational amplifier is constructed by a PNP bipolar transistor constitution as shown by FIG. 8. As a general disadvantage in the case of using such an input constitution, the frequency characteristic is worse than that of an NPN input stage constituting circuit (the function of the transistor is generally superior in the case of NPN). Further, when input or output is at a vicinity of 0 [V], there is a possibility that the transistor is brought into a saturated region and stability with regard to the frequency characteristic is deteriorated.
Further, although gain of the amplifiers Y1 and Y2 are adjusted to set by the variable resistors RV1 and RV2 connected to the inputs of the operational amplifiers Y9 and Y10, the frequency band and the phase are significantly varied by function of the operational amplifiers Y9 and Y10.
Further, the constitution of FIG. 7 is the circuit constitution specified to the 2LD-1PD element, the circuit constitution per se cannot be used as an APC circuit using a 1LD-1PD element or an APC circuit using two of 1LD-1PD elements and there is needed an APC circuit for separate use, which gives rise to an increase in the cost. For example, although in a printer apparatus, there is used a laser diode for scanning a printing drum, in a high grade machine, scanning is carried out at high speed and therefore, the machine is dealt with by a 2LD-1PD element or two of 1LD-1PD elements, which constitution is to be selected is determined by price and function of laser diode and in the case of a low grade machine, scanning may be carried out at low speed and the machine is dealt with by a 1LD-1PD element. Therefore, APC circuits need to prepare for different machines, which results in an increase in the cost.
Hence, according to the invention ,by connecting a current source to a collector of respective bipolar transistor constituting a current mirror for dividing monitor current of a light receiving element and providing bias current thereto, drive function of a post stage is compensated and attenuation of operational frequency band is restrained.
Further, reference voltage of a current-voltage conversion portion is set in accordance with an input range of an amplifier at a post stage and input to the post stage amplifier is fitted to an input range optimizing amplifying function thereof. Further, by providing a differential amplifier for setting gain at a gain adjusting portion for adjusting gain of output voltage, a variation of frequency band or phase caused by adjusting the gain is restrained.
Further, there is provided a switching portion for selectively supplying monitor current from a plurality of light receiving elements to a plurality of drive circuits for generating drive current to thereby enable selection of whether a plurality of drive circuits are controlled by monitor current. of a desired one of the light receiving element or controlled by monitor current of the individual light receiving element.